Proposal 289 prevents SENDME-flooding by requiring the other side to
authenticate the data it has received. But this data won't actually
be random if they are downloading a known resource. "No problem",
we said, "let's fell the empty parts of our cells with some
randomness!" and we did that in #26871.
Unfortunately, if the relay data payloads are all completely full,
there won't be any empty parts for us to randomize.
Therefore, we now pick random "randomness windows" between
CIRCWINDOW_INCREMENT/2 and CIRCWINDOW_INCREMENT. We remember whether we have
sent a cell containing at least 16 bytes of randomness in that window. If we
haven't, then when the window is exhausted, we send one. (This window approach
is designed to lower the number of rng checks we have to do. The number 16 is
pulled out of a hat to change the attacker's guessing difficulty to
"impossible".)
Implements 28646.
Fortunately, in 0.3.5.1-alpha we improved logging for various
failure cases involved with onion service client auth.
Unfortunately, for this one, we freed the file right before logging
its name.
Fortunately, tor_free() sets its pointer to NULL, so we didn't have
a use-after-free bug.
Unfortunately, passing NULL to %s is not defined.
Fortunately, GCC 9.1.1 caught the issue!
Unfortunately, nobody has actually tried building Tor with GCC 9.1.1
before. Or if they had, they didn't report the warning.
Fixes bug 30475; bugfix on 0.3.5.1-alpha.
For memeq and friends, "tor_" indicates constant-time and "fast_"
indicates optimized. I'm fine with leaving the constant-time
"safe_mem_is_zero" with its current name, but the "tor_" prefix on
the current optimized version is misleading.
Also, make the tor_digest*_is_zero() uniformly constant-time, and
add a fast_digest*_is_zero() version to use as needed.
A later commit in this branch will fix all the users of
tor_mem_is_zero().
Closes ticket 30309.
